Our invention relates to lamps in general and, in particular, to a lamp assembly capable of throwing a beam of light in a preassigned direction at a considerable angle to its optical axis. The lamp assembly in accordance with our invention lends itself to use as, typically, a supplemental high mounted stop lamp on motor vehicles.
As the name implies, supplemental high mounted stop lamps are additional lamps of a vehicular stop lamp system that are mounted high, and typically interiorly of the vehicle rear window, for giving a steady warning light through intervening vehicles to operators of following vehicles. Some motor vehicles today, passenger cars in particular, have their rear windows arranged a considerable angle out of the perpendicular from hydrodynamic considerations or design preferences. In mounting supplemental stop lamps interiorly on such a steeply slanting vehicle rear window, it is desired that their seating plane be parallel to the window. So mounted, the stop lamps must of course emit beams of light at a considerable angle to their seating plane.
A typical conventional supplemental stop lamp intended for such use includes a lamp body to be mounted with its axis oriented horizontally with its sloping front end held interiorly against the vehicle rear window. The lamp body has at its rear end a paraboloidal reflector for producing parallel rays of light from a bulb positioned at its focus. Mounted at the open front end of the lamp body is a generally planar lens which is laid parallel to the vehicle rear window and so at a great angle to the lamp axis. The lens has a multiplicity of diverging lens segments molded in one piece for diverging the parallel light rays both laterally and vertically, in order that the resulting beam may cover a sufficiently wide area to the rear of the vehicle.
We object to this prior art stop lamp by reason of first of all, its bulk. The stop lamp must have a vertical dimension, at right angles with its optical axis, of not less than a prescribed limit to give a required degree of beam intensity. Because of the slanting rear window, however, the desired vertical dimension has been gained only by correspondingly increasing the lens dimension in the height direction of the lamp parallel to the window. This in turn has required an increase in the axial dimension of the lamp body, resulting in the inconvenient bulging of the lamp body toward the interior of the vehicle.
In addition to such mechanical or dimensional difficulties, the prior art stop lamp has had an optical problem as well. The parallel rays of light produced by the paraboloidal reflector are rendered divergent as aforesaid by the multiple lens segments which are molded in one piece as the generally planar lens member mounted at the slanting front end of the lamp body. The slant of the lens member with respect to the optical axis of the lamp assembly has been such that no negligible proportion of the parallel light rays from the reflector has been reflected away therefrom, instead of traversing same thereby to be diverged for beam coverage over a greater area.
We have proposed in our U.S. patent application Ser. No. 733,573, filed May 13, 1985, an improved lamp assembly designed to remedy these problems. This prior suggested lamp assembly comprises a lamp body having a paraboloidal reflector for producing parallel light rays from a bulk mounted at its focus, and two generally planar lens members disposed one behind the other at the front end of the lamp body so as to be normal to the principal axis of the lamp assembly. The inner lens member functions to diverge the parallel light rays laterally (or vertically) whereas the outer lens member serves to diverge the incident rays vertically (or horizontally) and to reorient the rays in a predetermined direction at an angle to the lamp axis.
The above lamp assembly of our prior application offers definite advantages over the more conventional ones. Since the lamp axis is perpendicular to the two lens members, as well as to the vehicular rear window, the axial dimension of the lamp assembly can be reduced to a minimum. Further the parallel light rays produced by the paraboloidal reflector can all traverse the two lens members to be emitted in the desired direction without energy loss.
We have found some weaknesses in this prior application, however, arising from the use of the paraboloidal reflector. The paraboiloidal reflector reflects not only the light energy from its source but also the heat generated thereby. The result is the excessive temperature rise of the lens system, particularly the inner lens member, to the detriment of its essential qualities. The avoidance of this problem by spacing the lens system a greater distance away from the reflector is objectionable as the axial dimension of the lamp system would increase correspondingly. We also object to the paraboiloidal reflector because it is expensive; the total cost of the lamp assembly will be reduced substantially without it. From the standpoint of lamp design, too, the paraboloidal reflector determines the shape of the rear portion of the lamp body, which is the most conspicuous part of the stop lamp as seen from inside the motor vehicle. The elimination of the paraboloidal reflector will give greater latitude to the lamp designer.